1. Field of the Invention
This invention relates to a time-domain receive beamformer using digital signal processing techniques, i.e., analog to digital converters, digital memories, adders, multipliers, filters, etc., and more particularly, to a method and apparatus for digital receive beamforming in a medical ultrasound diagnostic system.
2. Background of the Invention
The objective of beamforming in a system is to form a narrow beam for improving reception of a signal arriving from a desired location, in the presence of noise and interfering signals from other locations. Beamforming can be performed during energy transmission or reception. This invention relates to the formation of beams during reception.
Beamforming is useful in a number of applications, i.e., radar, sonar, communications, geophysics, astrophysics, etc. The present invention concerns beamforming in ultrasound imaging. Using medical ultrasound imaging apparatus, anatomical structures within a body of a patient can be displayed and analyzed. The apparatus transmits sound waves of very high frequency (typically 2 MHz to 10 MHz) into the patient and then processes the echoes reflected from structures in the body being examined. The purpose of the apparatus is to display and/or analyze the return echoes. There are many types of displays used by medical ultrasound diagnostic apparatus, but probably the one most generally useful is a two-dimensional image of a selected cross-section of the anatomical structure being examined. This important mode of operation is called the echo or B mode. Using this mode of operation, a number of anatomical defects in a patient can be detected. Furthermore, the size of these defects can be more or less precisely determined. In this mode of operation all echoes from a selected cross-section are processed and displayed. The most critical operational parameter with respect to performance in this mode of operation is the size of the resolution cell. The size of the resolution cell can be decreased (thereby increasing resolution) by implementation of dynamic focusing and dynamic (matched) filtering. These techniques are easier to implement with a digital beamformer than with an analog beamformer.
In some clinical applications, anatomical defects can be relatively small and overshadowed by echoes reflected from larger anatomical structures. However, a small anatomical defect in or near a blood vessel may manifest itself by causing a relatively large change in the velocity of blood flowing in the vessel. It is known that a Doppler shift echo processing technique can be used for determining the velocity of a moving object. The display of Doppler shift for blood flow allows relatively small anatomical abnormalities to be more easily detected. This mode of operation, now commonly referred to as Color Flow, such as described in U.S. Pat. No. 4,800,891 issued to Kim, allows Doppler information about blood velocity to be gathered from large selected cross-sections of the anatomical structure. It is difficult, however, to acquire sufficient ultrasound data to develop an accurate high resolution blood flow image at a sufficiently high frame rate. In order to get more precise Doppler information about blood flow velocity from a small cross-section area, a Doppler processing technique such as known, for example, from an article by Halberg and Thiele published in the Hewlett-Packard Journal, pp. 35-40, June 1986, may be used. Using this technique it is possible to devote more time to a selected small area. The Doppler data is usually processed by FFT techniques and displayed by means of a spectrum. The Doppler data is also presented as an audio signal.
The quality of the beamforming has its greatest influence on the accuracy, resolution and other parameters of the forenoted modes of operation of the ultrasound imaging apparatus. A conventional beamformer electronically provides time delays to match the signal propagation delays of the ultrasound pressure field which is incident upon the ultrasound beamformer from a specific direction. This time-delay (or spatial processing) enhances the amplitude of the coherent wavefront relative to the background noise and directional interference. In an analog beamformer, this is done using analog delay lines and summing networks. These analog components restrict modern ultrasound diagnostic equipment in many different ways and are therefore undesirable. They are relatively expensive, unstable, and influenced by environmental conditions and age. Analog components also require careful manufacturing and assembly. The use of analog delay lines also limits the desired flexibility of modern ultrasound apparatus. Many compromises have to be made in an analog beamformer in order to support the previously mentioned major modes of operation. Furthermore, parallel processing, which is necessary for increasing the frame rates of real time ultrasound equipment, is very costly if the beamformer is implemented using analog processing techniques.
The increase of performance and reliability and decrease of cost of digital components makes digital beamforming a more promising alternative as compared to classical analog beamforming. Precision, stability and flexibility are the main advantages of digital signal processing techniques. The current standard digital circuitry can work at Nyquist rates exceeding 30 MHz. These sampling frequencies are high enough for RF sampling and temporal processing of modern ultrasound signals. However, the sampling rate required to properly match the propagation delays in a digital beamformer is several times greater than the Nyquist rate for accurate signal reconstructions, i.e., it is more than 100 MHz. These processing speeds, coupled with the required precision, are still above the performance levels of presently available analog-to-digital converters (ADCs). The remaining digital functions (e.g. other than the ADC's) can be performed at these speeds by parallel processing using standard digital components.
A method proposed by Pridham and Mucci, in an article published in Proceedings of the IEEE, Vol. 67, No. 6, pp. 904-919, June 1979, eases the high speed sampling requirement for ADCs in digital beamforming by the use of digital interpolation. The received echoes need only be sampled at an interval which satisfies or exceeds the Nyquist frequency, f.sub.0. The price for this reduction in ADC sampling rate is a corresponding increase in the digital processing requirements. The fine delay increments necessary for beamforming are developed using digital interpolation. In digital interpolation, the data is first padded with zeros (e.g., zeros interspersed with the data), which effectively increases the data rate. At a later point in the processing, digital filters are used to reduce the data rate to its original value. Pridham and Mucci proposed two alternative approaches. In the first, a pre-beamforming interpolation approach, the zero padding circuitry and interpolation filters for each receive channel are placed after the ADC, but before the beamforming circuitry. In the second, a post-beamforming interpolation approach, the interpolation filter is placed after the beamforming. Filtering after beamforming is possible because beamforming is a linear operation. In the first approach, signal processing requirements are not optimal, since an interpolation filter is required for each received channel. In the second approach, the digital processing required for the interpolation filtering is reduced as compared with the required processing of the first approach since filtering is done only once rather than for each channel. The digital processing requirements can be further reduced by incorporating the interpolation filter into the digital filters of the receiver circuits which follow the digital beamformer. However, the beamforming signal processing is still not optimal because the beamformer processing rates (i.e., those needed to generate the required time delays) are much higher than the signal Nyquist rate.
It is an object of the present invention to provide a method and apparatus for digital beamforming which minimizes the signal processing rates in order that a system can be built with digital circuitry working at the signal Nyquist rate. Incorporation of such a method or apparatus in an ultrasound diagnostic system will offer all the advantages of digital beamforming, i.e., flexibility of the various modes of operation, parallel channel beamforming, dynamic focusing, matched filtering, etc, while minimizing the signal processing data rate.